The field of the invention is a system and method for optimizing intensity-modulated radiation therapy (IMRT) delivery systems.
Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry that can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is, e.g., located in the gantry for generating a high-energy radiation beam for therapy. During treatment, this radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
The point of such therapy is to concentrate radiation on tumors or other target zones, but minimize radiation dosages applied to adjacent healthy tissue, especially certain parts of the body (e.g., the optic nerve) that are more sensitive to radiation. A radiation source directs radiation towards the target zone. By moving the radiation source along an arc over a period of time, the radiation is on the target during the entire movement along the arc. However, healthy tissue adjacent the tumor (such as between the tumor and source, and tissue past the tumor along the beam path) receive radiation for only a small portion of the time, different sections of healthy tissue being in the radiation path at different places along the arc. Additionally, the patient is moved (usually rotated about a vertical axis) to achieve the same effect (i.e., radiation stays on the target during the entire treatment time, but healthy tissue is only exposed for a small fraction of the treatment time). Thus, the total radiation applied to the target may achieve the desired result, but the reduced radiation applied to adjacent tissue avoids or minimizes damage to the healthy tissues.
An important factor in such radiation treatment is maintaining the beam from the radiation source on the target zone. Precise positioning of the radiation source relative to the patient is thus required. The time of treatment affects the accuracy of the beam. A longer treatment time increases the chances that the patient or portion of the patient will move. Therefore, a shorter period of treatment is generally preferable because the chances of movement occurring is reduced.
To control the radiation emitted toward an object, a beam shielding device, such as a plate arrangement or a collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the object. An example of a plate arrangement is a set of, e.g., four plates that can be used to define an opening for the radiation beam. A collimator is a beam-shielding device that could include multiple leaves, for example, a plurality of relatively thin plates or rods, typically arranged as opposing leaf pairs. The plates themselves are formed of a relatively dense and radiation impervious material and are generally independently positionable to delimit the radiation beam.
The beam-shielding device defines a field on the object to which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume that includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The dose delivered to the tumor can be increased if the amount of normal tissue being irradiated is decreased and the dose delivered to the normal tissue is decreased. Avoidance of delivery of radiation to the organs surrounding and overlying the tumor determines the dosage that can be delivered to the tumor.
The delivery of radiation by a radiation therapy device is prescribed and approved by an oncologist, a doctor specializing in cancer and its treatment. The prescription is a definition of, for example, a particular volume and the level of radiation permitted to be delivered to that volume. A therapist, however, normally carries out actual operation of the radiation equipment. When the therapist administers the actual delivery of the radiation treatment as prescribed by the oncologist, the radiation-emitting device is programmed to deliver that specific treatment. When programming the treatment, the therapist has to take into account the actual radiation output and has to adjust the dose delivery based on the plate arrangement opening to achieve the prescribed radiation treatment at the desired depth in the target.
Modern day radiation therapy of, e.g., tumors has two goals: eradication of the tumor and avoidance of damage to healthy tissue and organs present near the tumor. It is known that a vast majority of tumors can be eradicated completely if a sufficient radiation dose is delivered to the tumor volume; however, complications may result from use of the necessary effective radiation dose, due to damage to healthy tissue which surrounds the tumor, or to other healthy body organs located close to the tumor. The goal of conformal radiation therapy is to confine the delivered radiation dose to only the tumor volume defined by the outer surfaces of the tumor, while minimizing the dose of radiation to surrounding healthy tissue or adjacent healthy organs.
Conformal radiation therapy has been traditionally approached through a range of techniques, and typically uses a linear accelerator (xe2x80x9cLINACxe2x80x9d) as the source of the radiation beam used to treat the tumor. The linear accelerator typically has a radiation beam source that is rotated about the patient and directs the radiation beam toward the tumor to be treated. The beam intensity of the radiation beam is a predetermined, constant beam intensity.
Multileaf collimators, which have multiple leaf, or finger, projections which can be moved individually into and out of the path of the radiation beam, can be programmed to follow the spatial contour of the tumor as seen by the radiation beam as it passes through the tumor, or the xe2x80x9cbeam""s eye viewxe2x80x9d of the tumor during the rotation of the radiation beam source, which is mounted on a rotatable gantry of the linear accelerator. The multiple leaves of the multileaf collimator form an outline of the tumor shape as presented by the tumor volume in the direction of the path of travel of the radiation beam, and thus block the transmission of radiation to tissue disposed outside the tumor""s spatial outline as presented to the radiation beam, dependent upon the beam""s particular radial orientation with respect to the tumor volume.
Intensity modulated treatment is a specialized technique for radiation treatment. Usually, the beam-shielding device in intensity modulated treatment includes either (1) two pairs of opposing jaws or (2) a pair of jaws and a pair of opposing sets of multi-leaf collimator leaves. One pair of these jaws or the pair of multi-leaf collimator leaves move in the same direction at different speeds. This creates a sweeping opening for the radiation beam. Because the jaws (or leaves) are traveling at different speeds, the opening varies in size during the sweeping. Usually, elaborate speed control and thick jaws (or leaves) are needed for intensity modulated treatment. The speed control is needed for accurately defining the changing opening size. The thick jaws (or leaves) are needed because of a concern with radiation leakage. For example, due to the sweeping treatment, approximately three times the regular amount of radiation dose is needed to treat an area on a patient. Therefore, the radiation leakage for intensity modulated treatment is approximately three times greater than regular leakage. For example, a regular treatment of 100 monitor units (MU) of radiation results in approximately 0.1 MU of radiation leakage. With an intensity modulated treatment for the same field, 300 MU of radiation is required and results in 0.3 MU of radiation leakage.
Intensity modulated treatment usually utilizes a multi-leaf collimator. The multi-leaf collimator can be in addition to the jaws, replace a pair of jaws, or replace all of the jaws. This collimator is typically rotated around the patient during the radiation treatment to provide more accurate radiation coverage. The leaves in the collimator each have a motor and two sensors. The sensors monitor the position of each of the leaves. Unfortunately, standard multi-leaf collimators usually have radiation leakage of approximately 0.5% to 1.5%. When multi-leaf collimators are used with intensity modulated treatment, the deposited radiation leakage increases due to the required increase in radiation dose. This is an unnecessary exposure to healthy tissue.
As noted above, the delivery of radiation by a radiation therapy device is prescribed and approved by an oncologist but actual operation of the radiation equipment is normally done by a therapist. When the therapist administers delivery of the radiation as prescribed by the oncologist, the radiation-emitting device is programmed to deliver that specific treatment. When programming the treatment, the therapist has to take into consideration the actual radiation output and has to adjust the dose delivery based on the plate arrangement opening to achieve the prescribed radiation treatment at the target depth in the patient.
This adjustment can be made according to known calculations, but the therapist normally has to do them manually, which can lead to errors. In the context of radiation therapy, a miscalculation can lead to either a dose that is too low and is ineffective, or that is too high and dangerous.
Intensity-modulated radiation therapy (IMRT) is an advanced form of external beam irradiation that is often referred as three-dimensional conformal radiation therapy (3DCRT). It utilizes variable beam intensities across the irradiated volume that has been determined using computer optimization techniques. IMRT has the capability of generating concave dose distribution and of providing specific sparing of sensitive normal structures within complex treatment geometries. It represents the most significant technical advancement in radiation therapy since the advent of the medical linear accelerator.
Current IMRT delivery system consists of either using a physical compensation filter made of non-uniform thickness or a multi-leaf collimator (MLC) to obtain variable intensities.
In the case of MLC, a gap formed by each pair of opposing MLC leaves is swept across the target volume under computer control with the radiation beam xe2x80x98ONxe2x80x99 to produce the desired radiation intensity profile. Treatment times for IMRT can be very long because only a small part of the field is treated at a time in the case of MLC and a large amount of radiation intensity is absorbed in filter material in the case of a physical compensating filter. In IMRT it is not uncommon to have sharp gradients of dose distribution between target (or critical organ) and its surrounding region. A small amount of patient movement can produce large errors in dose delivery to target (or critical organ). Since there are more opportunities for a patient to move where treatment times are extended, it is always desirable to reduce treatment times as much as possible. Moreover, long radiation exposures result in higher doses outside the prescribed field due to radiation leakage from the treatment machine which is very difficult, if not impossible, to avoid. Accordingly, the focus of much current research in IMRT is to minimize the radiation xe2x80x98ONxe2x80x99 time by optimizing the leaf-sequencing algorithm.
Apart from various LINAC systems, and decades ago, Dr. Lars Leksell and others developed a radiation treatment device that is called the GAMMA KNIFE. The device consists of a hemispheric arrangement containing numerous Cobalt-60 sources. The radiation from each of these sources is collimated and mechanically fixed, with great accuracy, on a focal point at the center of the hemisphere. When a patient has a suitable lesion for treatment (such as an intracranial arteriovenous malformation), it may be precisely localized with another device called a stereotactic frame. Using the stereotactic apparatus, the intracranial target is positioned at the focal point of the GAMMA KNIFE. Since each of the numerous radiation pathways is through a different area of the brain, the amount of radiation to normal brain tissue is minimal. At the focal point, however, a very sizable dose is delivered which can, in certain cases, lead to obliteration of the lesion. This system has generally been quite expensive.
Although various of the prior designs and techniques have been useful in treating patients, there still are various problems. Treatment times are often relatively long. Further, matching up the radiation to a tumor or other target often has required complex algorithms. As discussed, radiation is often either applied in too low a dose for effectiveness on the target or too much radiation is applied to healthy tissue. Indeed, sometimes there is both too low a dose for effectiveness on the target and too much radiation is applied to healthy tissue. Scatter radiation is a problem with various systems. Some techniques have been quite expensive.
Accordingly, it is a primary object of the present invention to provide a new and improved method and system for radiation therapy.
A more specific object of the present invention is to provide multi-source radiation treatment.
A still further object of the present invention is to provide radiation treatment with reduced treatment times such that the chances of patient movement are reduced.
Yet another object of the present invention is to provide radiation treatment where the portion of applied radiation that is on a target is increased and the portion of the applied radiation on healthy tissue is reduced.
A further object of the invention is to avoid or minimize the problems in other techniques as noted above.
The present invention relates to a novel system and method for radiation delivery that improves the duty cycle to almost its theoretical limit. xe2x80x9cDuty cyclexe2x80x9d is defined as the ratio of useful radiation intensity incident on a patient to the total useful radiation intensity produced by the machine in a given time. The theoretical limit of duty cycle is unitary, i.e., 1.0, whereas a duty cycle in a typical IMRT is 0.25. The present invention provides a beam delivery system embodying an array that comprises multiple radiation sources. The radiation sources may either be miniature X-ray machines or isotope radiation devices depending upon the purpose of treatment. In current systems, Cobalt-60 source is the first choice. Each individual source contributes a dose to a small field segment on a patient. Since each source is capable of being individually moved in space, the multi-source machine (MSU) can provide spatially non-uniform beam intensity, i.e., intensity modulated beams depending on the distance between each source and the patient. Because multiple sources can move at the same time, beam delivery time is of much shorter durations than in conventional systems. Scatter radiation is also greatly reduced over that in conventional MLC because beam attenuation is relatively insignificant between source and patient.